The present disclosure relates generally to auxiliary power units, and more particularly to fuel systems for providing fuel during starting of auxiliary power units.
Auxiliary power units (APUs) are used on aircraft for main engine starting, to provide in-flight engine restart assistance, and to supply power and compressed air for ground and in-flight operations. APUs are typically gas turbine engines that include a compression section, a combustion section, and a turbine section. The compression section draws in and compresses ambient air. The compressed air is mixed with fuel and ignited in the combustion section to produce high-energy combusted air. The combusted air is expanded in the turbine section, which rotates to drive the compression section and to provide additional power through an output shaft. Typically a gearbox is mounted to the output shaft of APUs for driving accessory generators and various pumps. Fuel is sent to the combustion section of an APU by one or more fuel pumps. Commonly used fuel pumps are high pressure, fixed volume, gear-driven type pumps that require mechanical rotation of the engine to produce fuel flow. During a start up operation of the APU, gear-driven pumps rotate at speeds much lower than normal operating speeds. In order to deliver the required volume of fuel for initial ignition in the combustor, the gear-driven pump must be sized to have a large displacement. Once the APU is started and rotating at normal operating speeds, the displacement of the gear-driven pump provides an excess volume of fuel flow. Accordingly, the typical gear-driven pump is over-sized and provides capacity in excess of the needs of normal APU operation thereby adding unnecessary size and weight.